The present invention relates to a producing apparatus and a producing method for producing a carbon structure such as a carbon nanotube or a fullerene.
Fullerenes represented by C60 discovered in 1985 and carbon nanotubes discovered in 1991 exhibit unique electron-physical properties different from conventional carbon materials. For this reason, fullerenes and carbon nanotubes attract attention as new carbon allotropes different from graphite, amorphous carbon and diamond.
For example, fullerenes represented by C60 and C70 have various sorts other than C60 and C70. In any fullerene, a large number of carbon atoms are disposed in a spherical cage form so as to form one molecule. In addition, since fullerenes are soluble in organic solvents such as benzene, fullerenes are easy to handle. Fullerenes exhibit not only properties as superconductors and semiconductors but also high photo-functional effect. Thus, it is also considered that fullerenes are applied to electro-photographic sensitive materials. Further, by doping the inside of fullerenes with different kinds of elements, or by adding various chemical functional groups to the outside of fullerenes, effective physical properties as functional materials are expressed.
On the other hand, carbon nanotubes are new materials having only carbon as their constituent element similarly to fullerenes. It was discovered that carbon nanotubes had photo-functional effect and functions as semiconductors or the like. Thus, carbon nanotubes are expected to be utilized in any field of the electronics industry. Particularly, since carbon nanotubes can be formed into semiconductors and also into conductors by changing a method of atomic arrangement (chirality) slightly, more expectations are placed on the carbon nanotubes as nanometer-sized low-dimension electrically conductive materials and switching elements. In addition, carbon nanotubes attract attention also as field emission type electron sources and hydrogen storage materials. Further, there are attempts to use carbon nanotubes as probes in tunneling electron microscopes or atomic force microscopes.
In the related art, it has been known that fullerenes and carbon nanotubes can be produced in a resistance heating method, a method based on plasma discharge such as arc discharge using carbon rods as raw material, a laser ablation method, and a chemical vapor deposition (CVD) method using acetylene gas. However, there have been various debates on the details of the mechanism with which fullerenes and carbon nanotubes are produced in the method using arc discharge or laser ablation, and there is no unified interpretation at present.
As for production of fullerenes and carbon nanotubes, various methods aimed at mass synthesis have been researched. The resistance heating method devised in the cradle was a method in which front ends of two pieces of graphite were brought into contact with each other in rare gas, and a current ranging from several tens of A to several hundreds of A was applied thereto so as to heat and evaporate the graphite. However, since it is very difficult to obtain a sample by the gram in this method, the method is hardly used today.
The arc discharge method is a method in which graphite rods are used as a cathode and an anode so that arc discharge is generated in rare gas such as He or Ar to thereby synthesize fullerenes or carbon nanotubes. The temperature in the front end portion of the anode is increased to about 4,000° C. or higher by arc plasma caused by the arc discharge. Thus, the front end portion of the anode is evaporated so that a large amount of carbon radicals are produced. The carbon radicals are deposited on the cathode or the inner wall of an apparatus in the form of soot containing fullerenes or carbon nanotubes. When an Ni compound or an iron compound is included in the anode, the compound acts as a catalyst so that single-wall carbon nanotubes can be produced efficiently.
The laser ablation method is a method in which graphite is irradiated with pulsed laser such as YAG laser so that plasma is generated on the graphite surface to thereby produce fullerenes or carbon nanotubes. This method has a feature in that comparatively high purity fullerenes or carbon nanotubes can be obtained in comparison with those obtained in the arc discharge method.
In the chemical vapor deposition method, acetylene gas or methane gas is used as raw material so that high purity fullerenes or carbon nanotubes can be produced by the chemical decomposition reaction of the raw material gas. Recently, there has been also discovered a method in which carbon nanotubes are produced efficiently by irradiation with a beam of electrons upon a fluorine compound subjected to chemical treatment.
When graphite rods are used as electrodes in the arc discharge method, a large amount of electrons or ions present in arc plasma collide with the anode-side graphite rod. As a result, the temperature of the front end of the graphite rod increases to about 4,000° C. so that a large amount of carbon radicals, carbon ions and neutral particles are released. It is considered that carbon nanotubes are produced in the course where the carbon radicals, the carbon ions and the neutral particles adhere to the cathode or a chamber (the inner wall of the apparatus) or reattach to the anode side. However, since various complicated chemical reactions are produced in the arc plasma by the collision with excited ions or electrons, it is difficult to control the quantity or kinetic energy of carbon ions. Thus, a large amount of amorphous carbon particles and graphite particles are produced simultaneously together with fullerenes or carbon nanotubes so as to form soot in which those particles are mixed.
Therefore, when fullerenes or carbon nanotubes are to be used industrially, it is necessary to refine and separate fullerenes or carbon nanotubes. Particularly, carbon nanotubes are not soluble in any solvent. To refine carbon nanotubes, there have been proposed techniques such as centrifugal separation method, oxidization method, ultrafiltration process, and electrophoresis. However, the physical properties and chemical properties of carbon nanotubes are substantially equivalent to those of amorphous carbon or graphite particles produced as impurities. Thus, no separation/refinement method has been established for removing these impurities perfectly. In addition, due to many refinement steps carried out, there is a problem that the yield is extremely lowered, or, on the contrary, alkali metal or organic matter is mixed through an surface active agent used as dispersant. To solve such a problem, it is desired that carbon nanotubes whose purity is as high as possible, that is, carbon nanotubes not contaminated with graphite particles or amorphous carbon are synthesized in the stage of synthesizing the carbon nanotubes.
As described previously, graphite is used as an electrode when fullerenes or carbon nanotubes are produced in the arc discharge method. This electrode is evaporated by arc discharge so as to form arc plasma containing C+ and radicals such as C and C2, which become sources of fullerenes or carbon nanotubes. However, the arc plasma containing C+ and radicals such as C and C2 also become sources of graphite particles or amorphous carbon. The details of the conditions under which the arc plasma containing C+ and radicals such as C and C2 are formed into graphite particles or amorphous carbon and into fullerenes or carbon nanotubes when they are deposited on the cathode, still remain unknown.
Description will be made below on the related-art problem of how to increase the purity, particularly the purity of carbon nanotubes which is desired to be high.
As for the synthesis of high purity carbon nanotubes in the arc discharge method, there is an example reported by Journet et. al, in which a single-wall carbon nanotube with purity of about 80% was synthesized (C. Joumet et al., Nature Vol. 388, p. 756-758). However, the purity is not sufficient, and it is desired to synthesize higher purity carbon nanotubes.
As for the laser ablation method, there is a report about the synthesis of a high purity single-wall carbon nanotube (A. Thess et al., Nature Vol. 273, p. 483-487). However, only a small amount of carbon nanotubes can be obtained in the laser ablation method. Due to the inefficiency, the cost of carbon nanotubes is increased. In addition, the purity remains about 70% to 90%. This purity cannot be regarded as high enough.
The chemical vapor deposition method depends on chemical reaction occurring in the course of the pyrolysis of methane gas as raw material. It is therefore possible to produce high purity nanotubes. In the chemical vapor deposition method, however, the growth rate of carbon nanotubes is extremely low. Thus, due to the inefficiency, it is difficult to use the chemical vapor deposition method industrially. In addition, the structure of the nanotube produced thus is imperfect with many defects in comparison with a nanotube synthesized in the arc discharge method or the laser ablation method.